1. Field of Invention
The present invention relates to laser based systems and methods. More specifically, the present invention relates to eyesafe ladar transmitters.
2. Description of the Related Art
Current applications require highly accurate laser transmitters for high resolution ranging (one-dimensional profiling) and/or laser illumination for two-dimensional and three-dimensional sensing applications. For example, one-dimensional profiling allows for the target returns to be matched against a database to identify the target type. For two and three-dimensional sensing applications, a tight, highly accurate sensing pulse is transmitted and used to illuminate features of a target. The tight pulses reflect off of various surfaces of the target differently and reflect return pulses which are processed with sophisticated signal processing algorithms to yield more complete images of the target. Two-dimensional and three-dimensional imaging allows for a display of the target return data or an image of the target based on data from a stored database.
One of the main problems with multi-sensor eyesafe ladar systems is the prohibitively high cost of the transmitter component. Current approaches to eyesafe ladar transmitter design involve Optical Parametric Oscillator (OPO) shifting of the Nd:YAG laser to 1.5 micron.
For the above-noted sophisticated vibration sensing applications and other applications, separate laser transmitters have been required. Unfortunately, the use of multiple transmitters adds significantly to the cost and weight of deployment and would be impractical for many significant applications. That is, using separate transmitters for each sensor function necessitates complex beam-combining optics or requires multiple transmit apertures. In either case, the size and weight (and cost) are significantly increased. Producing both Q-switched and coherent mode-locked modes in a single Nd:YAG OPO would be complex and costly.
No approaches are known to exist for combining both high pulse energy Q-switched and coherent mode-locked functions in the same transmitter. Hence, there is a need in the art for a simple, accurate, low cost, efficient laser transmitter suitable for use in remote, long range, vibration sensing applications which may be implemented in a single laser transmitter capable of performing single point ranging, one-dimensional profiling and/or laser illumination for two-dimensional and three-dimensional sensing applications.
The need in the art is addressed by the multifunctional laser of the present invention. In a particular embodiment, the present teachings are implemented in a multifunctional laser which, in a first operational mode, outputs a mode-locked beam and, in a second operational mode, outputs a Q switched illumination beam. The inventive laser includes a resonant cavity; a gain medium disposed with the cavity; a first arrangement in communication with the medium for causing a Q-switched signal to be transmitted from the cavity; a second arrangement in communication with the medium for causing a mode-locked signal to be transmitted from the cavity; and a mechanism for switching between the first arrangement and the second arrangement.
In the illustrative embodiment, the switching mechanism consists of a 90 degree rotator, a beam block, and a polarizer. The first arrangement may be implemented with a Q switch. The second arrangement may be implemented with a quantum well absorber or an acoustic crystal.
Unlike the single mode laser transmitters that typify the prior art, the mode-locking mechanism of the present invention causes the laser to output energy at all modes within the gain profile in phase with one another. The result is a series of tight clean pulses which may be used for range resolved vibration and one-dimensional (high resolution ranging) applications.
Hence, in accordance with the present teachings, a single transmitter is provided which enables both functions using only a small mechanical switching mechanism to move one optic and a beam block. Both Q-switched and mode-locked beams emit from the same aperture with the same polarization, thus allowing common beam steering and shaping optics to be shared for both functions (e.g., only a single telescope).
In the illustrative embodiment, the laser is an erbium or erbium, ytterbium-doped, fiber pumped laser and the mode-locking mechanism is a passive quantum well absorber crystal or an active acoustic crystal mounted in the laser cavity. The high pulse energy, Q-switched mode can be used for 3D range imaging, and the coherent mode-locked mode can be used for RRV sensing and 1D target profiling. The invention allows for a single transmitter aperture with a common polarization, which means that the ladar beam-steering and telescope optics are shared between the multi-functions, thus reducing size, weight, and cost of the multi-sensor ladar system.